Increasingly strict legal requirements regarding the permissible pollutant emissions from motor vehicles in which internal combustion engines are fitted make it necessary to keep the emission of pollutants as low as possible during the operation of internal combustion engines. On the one hand, this can be achieved by reducing the pollutant emissions that are generated during the combustion of the air/fuel mixture in each cylinder of the internal combustion engine. On the other hand, exhaust-gas aftertreatment systems are used in internal combustion engines to convert the pollutant emissions that are generated during the combustion of the air/fuel mixture in each cylinder into harmless substances. In petrol engines in particular, three-way catalytic converters are used for this purpose as exhaust-gas catalytic converters. In order to achieve a high level of efficiency in the conversion of the pollutants, such as carbon monoxide, hydrocarbons and nitrogen oxides, the fuel/air ratio in the cylinders must be precisely set. Furthermore, the mixture must exhibit a predetermined fluctuation upstream of the catalytic converter, i.e. a specific operation of the internal combustion engine is required when there is both a lean mixture and a rich mixture in order to bring about the filling and emptying of an oxygen tank of the exhaust-gas catalytic converter. As oxygen is taken in, nitrogen oxides in particular are reduced, whereas as it is expelled oxidation is encouraged and furthermore the deactivation of portions of the exhaust-gas catalytic converter by stored oxygen molecules is prevented.
As already mentioned, a three-way catalytic converter can only convert pollutants optimally if the fuel/air ratio λ is within a range around λ=1. The reason for this is that only fuel/air ratios of this type generate an exhaust-gas composition in which the oxygen that is released during the reduction of the nitrogen oxides is sufficient to nearly completely oxidize the HC and CO proportions of the exhaust-gas to CO2 and H2O. In an internal combustion engine with a three-way catalytic converter, the mixture generation is therefore regulated to a nominal value of λ≈1 by a so-called lambda control. In order to compensate for short-term fluctuations in the fuel/air ratio, the catalytic converter also contains a layer which can store oxygen for a short time and either bind it or release it as required.
In the reference book “Handbuch Verbrennungsmotoren”, ed. Richard von Basshuysen/Fred Schäfer, 2nd edition, June 2002, Friedrich Vieweg & Sohn Verlagsgesellschaft mbH, Brunswick/Wiesbaden, page 641 ff., an exhaust-gas aftertreatment system for an internal combustion engine, specifically a petrol engine, is described with a three-way catalytic converter with regulation of the air/fuel ratio λ=1 and an oxygen tank with a catalytic coating with cerium, which possesses the property of being able to store oxygen and release it again.
In order to ensure that the maximum pollutant emissions that have been appropriately predetermined are not exceeded, legal regulations require the frequent diagnosis of components of the exhaust-gas tract of the internal combustion engine. The oxygen storage capacity of the exhaust-gas catalytic converter must be diagnosed in this way for example.
The oxygen storage capacity of the catalytic converter is tested using so-called OSC catalytic converter diagnostics (OSC=oxygen storage capacity). A rich/lean oscillation of the mixture is effected by pre-controlling the lambda regulation. The oscillation is compensated for by the oxygen storage capacity of the catalytic converter if the latter is in good order. The signal from the lambda sensor that is arranged downstream of the catalytic converter will only exhibit an oscillation with a small amplitude in this case. If the catalytic converter has aged, its oxygen storage capacity will be reduced and the signal from the lambda sensor downstream from the catalytic converter will exhibit a clear oscillation in response to the stimulus of a rich/lean oscillation at the catalytic converter inlet.